The present invention generally relates to an active four-wheel steering system for motor vehicles, and more particularly to a four-wheel steering system providing improved vehicle handling.
In German Published Unexamined Patent Application (DE-OS) 31 55 618, motor vehicles are disclosed having a four-wheel steering system which comprises steerable front and rear wheels, the rear wheels being controllable as a function of driving condition quantities which are calculated and/or measured and then fed to an electric control unit. For this purpose, the rear wheels are connected with an additional adjusting device for the control of the rear wheels which, corresponding to the actual driving condition quantities, can be adjusted in a certain relationship to the front wheels.
Thus, it is an object of the invention to provide a four-wheel steering system having front wheels actuated by a steering wheel and rear wheels controlled by an electronic control unit as a function of driving conditions, the rear wheels being adjusted with respect to the front wheels in a variable wheel steering angle relationship by means of which an improved vehicle handling is achieved in comparison to vehicles with conventional front-wheel steering systems.
According to the invention, these and other objects are achieved by generating a signal representative of the lateral acceleration (ay) of the vehicle from the driving speed (v) and front wheel steering angle (.delta..sub.f) which, together with the driving speed (v) is employed to calculate the rear steering angle .delta..sub.r as a function of the vehicle speed (v) and the front wheel steering angle (.delta..sub.f) so that the sideslip angle of the vehicle is reduced. Other advantageous characteristics will be evident as the description of the present invention proceeds.
Advantages achieved by the invention include:
improvement of vehicle handling at high speed (gain in directional control) and improvement of maneuverability at low speeds;
precise reaction of the vehicle to steering angle input by means of short response times during the buildup of a lateral acceleration;
better controlling of disturbance variables (cross wind, pavement irregularities) by the driver;
no delay of motion or oscillations of the vehicle following the end of steering maneuvers;
more indirect steering behavior at higher speeds;
high damping of the natural yaw frequency;
influencing of the vehicle reaction after powering-off and braking maneuvers;
small number of vehicle condition quantities that must be measured directly.
States of motion, in which the motor vehicle approaches the limits of directional control or is even without directional control, are generally characterized by the occurrence of larger sideslip angles. Further, the driving behavior of the motor vehicle depends considerably on the size of this sideslip angle or the change of the sideslip angle during transient cornering. The buildup of the sideslip angle after a steering maneuver results in a time delay in the rise of normal acceleration on the path and thus in a change of course. A steering wheel change is therefore not immediately followed by the desired change of course. Inversely, the reduction of the sideslip angle, during the transition to straight-ahead driving, leads to changes of course, although the driver, is no longer steering. This effect is felt as an "afterpushing" delay of motion during which the rear of the vehicle tends to swing forward and round towards the front of the vehicle. In addition, the sideslip angle represents a measurement for the directional control of the vehicle. Any reduction of the sideslip angle means that there is a gain in directional control.
The invention therefore has a particular object of adjusting the wheels in such a manner that the sideslip angle is reduced or that a reduction toward zero is caused. Since a direct measurement of the sideslip angle is expensive, it is determined from other quantities, for the purpose of which the vehicle is observed during steady state driving in a circle.
By means of the geometrical relationships of a vehicle during a steady-state driving in a circle at low speed (FIG. 2) the following is obtained by approximation for a front steering angle: EQU .delta..sub.fo =(1/R)+.delta..sub.r ( 1)
and for the sideslip angle EQU .beta..sub.o =(1 .sub.h/ R)+.delta..sub.r ( 2)
wherein:
______________________________________ 1: wheel base .sup..delta. r: rear wheel steering angle .sup.1 h: distance between .sup..beta. o: geometrical sideslip the center of angle gravity(s) of the the vehicle and the rear axle R: radius .sup..delta. fo: geometrical front wheel steering angle ______________________________________
At higher speeds or lateral accelerations, the front wheel steering angle .delta..sub.f as well as the sideslip angle .beta. depend on the understeer/oversteer properties of the motor vehicle. The understeer/oversteer properties, in FIG. 3, are entered over the lateral acceleration a.sub.y of the vehicle. The characteristic curves to be measured for the front wheel steering angle front .delta..sub.f and the sideslip angle .beta. may be represented with good precision by means of polynomials of the second degree. Since the lateral acceleration a.sub.y of the vehicle is to be calculated from these polynomials, quadratic equations must be determined in the solution. As an example, the equations are linearized (FIG. 3), so that the front wheel steering angle .delta..sub.f is calculated from the equation: EQU .delta..sub.f =.delta..sub.fo +C.sub.1 .multidot.a.sub.y ( 3)
and the sideslip angle is calculated from the equation EQU .beta.=.beta..sub.o +C.sub.3 .multidot.a.sub.y ( 4)
where C.sub.1 and C.sub.3 are constants.
However, it is also possible to process the quadratic equations in a corresponding control apparatus. The radius R of the circle which is driven by the vehicle is calculated by approximation from the equation EQU R=v.sup.2 /a.sub.y ( 5)
When equations (1), (3) and (5) are used, the front wheel steering angle (.delta..sub.f) for a four-wheel steered vehicle is obtained from: EQU .delta..sub.f =a.sub.y .multidot.((1/v.sup.2)+C.sub.1)+.delta..sub.r ( 6)
and the sideslip angle (.beta.) from equations (2), (4) and (5) EQU .beta.=a .sub.y .multidot.((1.sub.h /v.sup.2 +C.sub.3)+.delta..sub.r ( 7)
The only unknown quantities in equations (6) and (7) are still the lateral acceleration a.sub.y and the rear wheel steering angles .delta..sub.r. The driving speed v is available as a measured signal, and the front wheel steering angle .delta..sub.f is measured, for example, by means of a steering wheel angle sensor.
Under the condition that the sideslip angle .beta. is reduced to 0 degrees, the following is obtained for the lateral acceleration: ##EQU1## and for the rear wheel steering angle EQU .delta..sub.r =-(.sup.1 h/.sub.v 2+C.sub.3).multidot.a.sub.y ( 9)
In function calculation task apparatus, the lateral acceleration a.sub.y and the rear wheel steering angle .delta..sub.r are calculated from equations (8) and (9) from the input quantities driving speed v and front wheel steering angle .delta..sub.f.
In addition, according to the teachings of the present invention, as shown in FIG. 4, a transmission ratio k of the rear wheel steering angle to the front wheel steering angle can be shown over the vehicle speed v, in which case, in the speed range of more than, for example, 55 km/h, the rear wheels are steered in the same direction, making the steering behavior of the vehicle more indirect. In contrast, at a low speed, the rear wheels are steered in the opposite direction of the front wheels and improve the maneuverability of the vehicle.
In accordance with certain preferred embodiments of the present invention, by means of the adjustment of the steering angles at the rear wheels, the vehicle reaction can be influenced during the driving maneuvers powering off and braking from steady-state driving in a circle. This vehicle reaction depends on the actual lateral acceleration and longitudinal deceleration (FIG. 6 and 7). The lateral acceleration is known from equation (8), while the longitudinal deceleration is determined from a differentiating of the vehicle speed.
By means of the characteristic curve of FIG. 6, an additional rear wheel angle .delta.'.sub.r,BL is first determined as a function of the lateral acceleration. This steering angle is multiplied by an amplification factor V.sub.BL (FIG. 7) as a function of the longitudinal deceleration. An effective steering angle: EQU .delta..sub.r,eff. =.delta..sub.r +V.sub.BL.multidot. .delta.'.sub.r,BL
is obtained which is composed of the sum of the rear wheel steering angles for the lateral and longitudinal dynamics.
When adjusting the rear wheels with respect to the front wheels as a function of determined driving condition quantities, it was found, among other things, that a vehicle with four-wheel steering has more direct steering characteristics when the radii are small and the speeds are low. In contrast, more indirect steering characteristics must be expected in the case of large radii and high speeds. This means for the driver that the vehicle does not have to be steered as carefully at high speeds, and that the vehicle reacts less violently to steering angle inputs.
Four-wheel steering also has an advantageous effect on the driving behavior of the vehicle during lane changes, because steering wheel angle changes, and changes of course are coupled with the shortest time delay. The vehicle reacts spontaneously in comparison to vehicles with front wheel steering. The phase angles of the yaw velocity and lateral acceleration are almost identical; the turning of the vehicle therefore takes place simultaneously with the change of course, whereas, in the case of conventional front-wheel steered vehicles, a change of course takes place only after the change of the steering wheel angle.
During a powering-off, while driving in a circle, a vehicle with front wheel steering has positive yaw velocity differences over the whole lateral acceleration range. As a result, the vehicle disadvantageously tends to turn into the circle, i.e. oversteers. By comparison, a vehicle with four-wheel steering acts neutral up to a relatively high lateral acceleration. At higher lateral accelerations, this vehicle will also be oversteering. A countersteering with the rear axle will no longer be sufficient, because the adhesion potential is utilized. However, the yaw response is reduced considerably. Up to a relatively high lateral acceleration, the four-wheel steered vehicle will move exactly along the desired path. The otherwise normal load change reaction is compensated.
When the vehicle is braked when driving in a circle, the directional control as well as the stability are improved considerably by means of the four-wheel steering. Not only the vehicle reaction during the braking operation can be reduced, but higher braking decelerations during cornering can also be achieved. Since a vehicle with front wheel steering reduces the path radius as a result of the braking maneuver, higher lateral accelerations are obtained which result in a locking of the front wheel which is on the inside during the cornering. As a result, the vehicle loses its maneuverability and turns out of the circle.
Thus, in accordance with certain preferred embodiments of the present invention, by means of a targeted time delay as a result of the interposition of a so-called delay-time element, it is advantageously achieved that either the input signal for the front steering wheel angle or the input signal for lateral acceleration is fed to the corresponding function calculation task apparatus in a delayed manner, whereby the rear wheel steering angle is implemented with respect to the front wheel steering angle with a time delay.
Since the longitudinal deceleration takes place before the powering off reaction, the value for the longitudinal deceleration must be delayed in time. For this purpose, a low-pass filter is provided to which the input signal of the longitudinal deceleration is fed. This low-pass filter provides a soft countersteering movement of the rear axle and avoids an abrupt adjustment of the rear wheels.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.